1. Technical Field
The present invention relates to a transmitter used in communication devices such as mobile phones and wireless LAN devices, and to a communication device. In particular, the present invention relates to a transmitter that can operate with low distortion and high efficiency, and to a communication device using the transmitter.
2. Background Art
It is required for communication devices such as mobile phones and wireless LAN devices to secure the precision of output signals and operate with low power consumption. A transmitter that can operate with low distortion and high efficiency is needed for such communication devices.
FIG. 12 is a block diagram showing the configuration of a signal generation section 500 used for a conventional transmitter disclosed in Patent Literature 1. In FIG. 12, the conventional signal generation section 500 includes a signal output section 501, digital filters 502 and 503, and an ASIC (application specific integrated circuit) 504. The signal output section 501 outputs an I signal (in-phase signal) and a Q signal (quadrature phase signal). The I and Q signals are inputted to the digital filters 502 and 503, respectively. The digital filters 502 and 503 remove spurious components from the I and Q signals, and output the resultant signals as It and Qt signals, respectively. The It and Qt signals are inputted to the ASIC 504.
The ASIC 504 calculates an amplitude signal AM, based on the It and Qt signals. Further, the ASIC 504 generates, by dividing the It and Qt signals by the amplitude signal AM, Ip and Qp signals which have been normalized. The ASIC 504 outputs the amplitude signal AM represented by (expression 1) and the Ip and Qp signals represented by (expression 2).[Expression 1]AM=(It2+Qt2)1/2  (expression 1)[Expression 2]Ip=It/AMQp=Qt/AM  (expression 2)
Although Patent Literature 1 does not disclose any example in which the signal generation section 500 is applied to a polar modulation circuit, it is also conceivable to apply the signal generation section 500 described above to a polar modulation circuit. Hereinafter, a polar modulation circuit to which the signal generation section 500 is applied is referred to as a conventional transmitter 510. It is conceivable to configure the conventional transmitter 510 as shown in FIG. 13. In FIG. 13, the conventional transmitter 510 includes the signal generation section 500, LPFs 511 to 513, a quadrature modulator 514, and an amplitude modulator 515.
As described above, the signal generation section 500 outputs an amplitude signal AM and normalized Ip and Qp signals. The amplitude signal AM is inputted to the LPF 511. The LPF 511 removes a spurious component from the amplitude signal AM. The amplitude signal AM, from which the LPF 511 has removed a spurious component, is inputted to the amplitude modulator 515.
The Ip and Qp signals are inputted to the LPFs 512 and 513, respectively. The LPFs 512 and 513 remove spurious components from the Ip and Qp signals, respectively. To be specific, when the Ip and Qp signals are digital signals, the LPFs 512 and 513 remove quantization noise from the Ip and Qp signals, respectively. When the Ip and Qp signals are analog signals, the LPFs 512 and 513 remove image signals from the Ip and Q1, signals, respectively.
The Ip and Qp signals, from which the LPFs 512 and 513 have removed spurious components, are inputted to the quadrature modulator 514. The quadrature modulator 514 performs quadrature modulation on the Ip and Qp signals inputted to the quadrature modulator 514 via the LPFs 512 and 513, and outputs a radio frequency signal Pi. The radio frequency signal Pi is inputted to the amplitude modulator 515. The amplitude modulator 515 performs amplitude modulation on the radio frequency signal Pi with the amplitude signal AM and outputs a transmission signal Po.